On Sat, Nov 09, 2002 at 11:50:17AM +0900, Jun-ichiro itojun Hagino wrote:
> >> if not, is it acceptable if we test the feature in
> >> configure.in and use it if available?
> >I'd say so (as long as this doesn't involve peppering the code with
> >#ifdefs - if the feature *isn't* available, you still have to use the
> >EXTRACT macros),
>
> i was thinking about something like this:
>
> #if GCC
> #define __ATTRIBUTE_PACKED__ __attribute__((packed))
> #else
> #define __ATTRIBUTE_PACKED__
> #endif
>
> struct foo {
> u_int32_t bar;
> u_int32_t baz;
> } __ATTRIBUTE_PACKED__;
Or just use "__attribute((packed))", as "interface.h" does
#ifndef HAVE___ATTRIBUTE__
#define __attribute__(x)
#endif
so that __attribute__ is no-oped out if the compiler doesn't support it.
(Yes, this breaks if there's a compiler that supports it in a
non-GCC-compatible fashion, but there's already stuff in tcpdump that
depends on __attribute__ either not existing or working as it does in
GCC.)
Presumably then you'd do
#if defined(LBL_ALIGN) && !defined(HAVE___ATTRIBUTE__)
or something such as that in "extract.h", so that raw "ntohs()" and
"ntohl()" will be used if either
1) the platform supports efficient unaligned memory accesses
(LBL_ALIGN isn't defined)
or
2) the platform supports __attribute__((packed)) so that the
compiler will generate reasonably efficient code to fetch
unaligned stuff
and then convert references to use the EXTRACT_ macros.
> >although note that "__attribute((packed))", at least
> >according to the GCC 3.2 manual:
> >
> >
>http://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/Variable-Attributes.html#Variable%20Attributes
> >
> >changes the layout of the structure, so you'd have to add padding to
> >structures that lack it.
>
> if they don't have padding, they do not work right even now.
I.e., a structure
struct foo {
u_int32_t a;
u_int8_t b;
u_int32_t c;
};
won't work as a packet format description - you have to use
struct foo {
u_int32_t a;
u_int8_t b;
u_int8_t pad[3];
u_int32_t c;
};
? If so, are there any structures of the sort that won't work?
-
This is the TCPDUMP workers list. It is archived at
http://www.tcpdump.org/lists/workers/index.html
To unsubscribe use mailto:tcpdump-workers-request@;tcpdump.org?body=unsubscribe
